CVD film forming processes are generally accomplished with all dispersion heads (D/Hs) filled with wafers, in the manner that a wafer to be deposited is loaded on each D/H. This is depicted in FIG. 1 and FIG. 2. In the event there are five D/Hs in a chamber, dummy wafers D1-D4 are moved from a dummy wafer cassette 1 to an elevator successively using a cold arm 12 and are loaded on each D/H E-B using a load susceptor 13. Then, a first slot wafer W1 is moved from a run cassette 3 to the elevator and is loaded on D/H A. Consequently, all five D/Hs are loaded with wafers. This is an initial wafer loading step and is shown in FIG. 1.
The wafer loading step is followed by a CVD processing step for depositing a material of predetermined thickness on the wafers. The thickness of material deposited in each step in the deposition process is determined by the quotient of 100% divided by the number of the D/Hs on the supposition that the thickness to be finally obtained is 100%. That is to say, the thickness to be obtained by a deposition process is 100%/5=20% in the above embodiment. After an initial CVD deposition step, the wafers in the chamber are moved to the next D/Hs. At this time, the first loaded dummy wafer is unloaded from D/H E through the elevator and a second slot wafer is loaded from the run cassette 3. If the steps of depositing films on the wafers, moving the wafers to the next D/Hs, unloading the wafer moved onto D/H E and loading a new slot wafer on D/H A are repeated, the dummy wafers D1-D4 which are unloaded will have received only 20%, 40%, 60% and 80% of the material deposited on the slot wafers, respectively. The unloaded wafers including the dummy wafers D1-D4 stay at a cooling station 10 for approximately 30 minutes and are stored in their respective original positions in the cassettes 1, 3. In such manner, deposition on all of the wafers in the run cassette 3 is completed.
In the case of a run cassette 3 with 25 wafers, the deposition process is executed with a 25th slot wafer loaded on the D/H A. When the deposition process is completed, the wafers are moved to the next D/Hs. Here, a 21st slot wafer (loaded on D/H E) is unloaded through the elevator. Since there are no more slot wafers left in the run cassette at this time, the unloaded dummy wafers are then reloaded on the D/Hs A-D as the 25th slot wafer moves from D/H A to D/H E. The case where D/H E is occupied by the 25th slot wafer is illustrated by FIG. 2. This is followed by a clean-up process and the cleaned wafers are then unloaded.
Unfortunately, this conventional method has several limitations. First, it requires the use of dummy wafers which may be in short supply if high wafer reliability, which is desired, is achieved. In addition, dummy wafers typically need to be changed in a range between about once a week and once every three weeks. Second, dummy wafers may be prone to cracking and particles generated by cracked dummy wafers may contaminate other wafers during processing and reduce wafer yield and reliability. Third, the use of dummy wafers limits that ability to achieve continuous processing of wafers because the steps of using dummy wafers must be controlled by a human operator instead of by computer. This reduces efficiency and increases cost.
Thus, notwithstanding the above described method which requires the use of dummy wafers, there continues to be a need for improved methods of depositing films on semiconductor wafers.